A few changes:

1. made a more sensible 'x_max' for interactions
2. fixed mex and xls files so to remove warnings, plus added a fix to the
dimentions of celldim
3. Made 'make_movie' a little bit more sensible. Main change, now the background PES
of the unitcell is reduced (digitized) before replicating it to tile the
supercell.